1. Technical Field
Embodiments described herein relate to a wireless communication device and a wireless communication system.
2. Related Art
A star network is a typical example of the configuration of a wireless network. FIG. 7 shows the configuration of a star network in which a device A 500a located in the middle performs wireless communication with a device B 500b, a device C 500c, and a device D 500d. As an example, the case of transmitting the data from the device A 500a to the device B 500b, the device C 500c, and the device D 500d by time division multiple access (TDMA) will be described below.
In the time division multiple access method, a fixed communication period is divided into short times (hereinafter, referred to as “communication slots”) and these are assigned for communication between devices (nodes) in order to multiplex communication between the devices.
FIG. 8 is a view showing an example of allocation of communication slots in the wireless network shown in FIG. 7. In FIG. 8, “A→B” indicates a communication slot used for data transmission from the device A 500a to the device B 500b. This is the same as for “A→C” and “A→D”.
Incidentally, as one of the effective methods for ensuring the communication reliability in wireless communication in which the communication path is unstable compared with that in cable communication, retransmission of data may be considered. The data retransmission is a processing of transmitting the transmission data whose communication has failed again using a communication slot set for retransmission. In the example shown in FIG. 7, a retransmission slot “Retry” which is a communication slot for data retransmission is prepared in addition to the communication slots of “A→B”, “A→C”, and “A→D”.
The retransmission slot “Retry” is used to transmit the data again when communication of “device A→device B”, “device A→device C”, and “device A→device D” has failed, and is shared by “device A→device B”, “device A→device C”, and “device A→device D”.
Generally, the number of assigned retransmission slots is determined based on the error rate of communication between devices so that the constant communication quality can be maintained in consideration of retransmission. The communication quality is ensured by reducing the number of retransmission slots when the communication situation is good, that is, when the error rate is low and on the contrary, increasing the number of retransmission slots when the communication situation is bad, that is, when the error rate is high.
It is determined whether or not the communication has succeeded by transmission of arrival acknowledgement (ACK) from a receiver to a transmitter, for example. In this case, the transmitter sets itself to a receivable state immediately after the data transmission and waits for ACK from the receiver. Then, when the ACK is received within a given time, the transmitter determines that the communication has succeeded. On the other hand, when the ACK cannot be received within a given time, the transmitter determines that a communication error has occurred and performs data retransmission using a retransmission slot. Moreover, in FIG. 8, processing of ACK is assumed to be performed in each communication slot, and thus the illustration is herein omitted.
For example, see JP-A-2008-263511.
Generally, the number of assigned retransmission slots is set such that the target data arrival rate can be ensured according to the communication environment. As examples of the setting method, there is a method of setting the number of retransmission slots in advance after estimating the error rate by performing environmental research before operating the wireless network or there is a method of setting the number of retransmission slots dynamically according to the error rate while actually operating the wireless network.
Usually, the communication error rate is not constant but changed depending on various factors, such as the influence of noise from the outside or an obstacle. In addition, an increase in the communication error caused by an obstacle, a noise source, or the like between devices is continued until the obstacle or the noise source is removed.
The unsent data is managed using a transmission queue. However, if a communication situation between certain specific devices becomes worse and a communication error occurs very frequently, a number of retransmission data addressed to the specific device are stored in the transmission queue. This may have an adverse effect on the retransmission data addressed to other devices.
FIG. 9 is a schematic view showing the situation of a transmission queue and communication slots of the device A 500a when the situation of communication from the device A 500a to the device D 500d becomes worse. In FIG. 9, a communication slot with an x-mark (cross-mark) indicates a communication slot in which communication has failed, and it is assumed that all communication of “A device→D device” has failed and each of communication of “device A→device B” and communication of “device A→device C” has failed once. FIG. 9 shows a state that no retransmission data can be transmitted even in the retransmission slot “Retry” and data remains as unsent data in the transmission queue at time T.
As shown in FIG. 9, the transmission queue is occupied by a number of retransmission data of “device A→device D”, and this has an adverse effect on data retransmission of “device A→device B” and data retransmission of “device A→device C”. In addition, since the size of the transmission queue is limited, the retransmission data exceeding the capacity of the transmission queue is discarded according to given rules. For this reason, the transmission data between other devices may be discarded due to the retransmission data between specific devices.
The number of retransmission slots is set such that the target data arrival rate can be ensured even in a state where the communication situation between specific devices becomes worse. In the method of setting the number of retransmission slots in advance after estimating the error rate, however, the number of retransmission slots becomes insufficient if the communication situation between the specific devices becomes worse than the initial estimation. As a result, the communication quality between other devices is adversely affected, and the target data arrival rate cannot be ensured.
In contrast, in the method of setting the number of retransmission slots dynamically according to the error rate, it is possible to increase the number of retransmission slots according to deterioration of the communication situation between specific devices. However, since there is an upper limit in the number of assignable retransmission slots, it is not possible to sufficiently prevent the deterioration of the communication situation between specific devices from imposing an adverse effect on the communication quality between other devices.